The present invention relates to a stabilized polyorganosiloxane composition. More particularly, the invention relates to a process of making a coating, a light bulb coated with the coating and a process of applying the coating to a light bulb.
The globe of a light bulb is thin and frail and is vulnerable to breaking on impact. Additionally, some recent bulb designs include an inner filament tube under positive pressure. The filament tube can rupture and shatter an outer globe. The bulb can be coated on the outside with an elastomeric polyorganosiloxane coating to improve impact resistance and to prevent scattering of glass shards in the event the bulb does break. A polyorganosiloxane coating advantageously combines high impact strength, transparency, colorlessness and low cost. However, most polyorganosiloxane coatings lack thermal stability above 180° C. This severely restricts effective service life of polyorganosiloxane coated bulbs, which typically encounter operating temperatures well in excess of 180° C. While iron oxide or copper containing compounds can be added as thermal stabilizers, these compounds impart significant opacity and/or color to the bulb coating.
There is a continuing need for a heat stabilizer for a polyorganosiloxane to coat a bulb that will extend service temperature and composition life and will preserve the transparent and colorless characteristics of an uncoated bulb.